Mixed Venous and Arterial Pco₂

Abstract

The rebreathing method of measuring oxygenated mixed venous Pco₂ (Pv̄co₂) was originally introduced as a bloodless way to estimate arterial Pco₂ (Paco₂). It has become common practice to subtract 6 mm Hg from the Pv̄co₂ to obtain the Paco₂ but there are many circumstances in which this leads to an overestimate of the Paco₂. Measurements of Pv̄co₂ and Paco₂ in 19 patients have shown that a better approximation to Paco₂ under normal conditions of cardiac output and arterial O₂ saturation is Paco₂ = 0·8 Pv̄co₂. These studies also showed that the Pv̄co₂ — Paco₂ difference may be much wider, particularly in the presence of arterial unsaturation and a low cardiac output.

The factors governing the venoarterial Pco₂ difference are reviewed and their magnitude is calculated to emphasize the complementary roles of measurements of Pv̄co₂ and Paco₂ in the assessment of patients with cardiorespiratory disease.